Marine seismic data is typically acquired by hydrophones in a cable or cable array that is towed behind the acquisition vessel and kept several meters below the sea surface. The hydrophones record both waves which travel directly from the earth to the receivers (the “primary” signal) and those same waves after they have been reflected at the sea surface and arrive after a time delay (the “ghost” signal). The ghost signal interferes with the primary signal constructively or destructively, giving a distorted signal. This is particularly true when the source and receiver are relatively close together, as the time delay of the ghost is not sufficient to separate it from the primary signal. Ghosts can occur relative to the source (“source ghosts”) and relative to the receiver(s) (“receiver ghosts”).
Because ghosts distort the primary signal, they reduce the value of the resulting seismic data. Therefore, it is common to undertake various techniques to reduce the effect of ghosts. Mitigation or elimination of ghosts from the desired data is called “deghosting.”
If data are gathered simultaneously using both hydrophones (pressure sensors) and geophones (accelerometers), the difference between the two datasets can be used to deghost the data. If two types of sensors are not available, however, other deghosting techniques must be applied.
Some conventional seismic processing techniques attempt to deghost the data using an assumption that the waves are traveling in a near vertical direction as they approach the instruments. However, waves can and do travel at angles through the water, due to reflector dip and/or shot-receiver offsets. The nature of the distortion varies, and the vertical deghosting procedure doesn't fully remove it. The problem becomes more significant as the depth of the receivers increases. Thus, it is increasingly desirable to provide a deghosting technique that takes into account the propagation angle of the incoming seismic signal.